1. Technical Field
The present invention relates generally to an apparatus and a method for using electron beams to microscopically inspect the surface of an object, and more particularly to inspect layers in a semiconductor device.
2. Discussion of the Prior Art
A variety of methods have been used to examine microscopic surface structures of semiconductors. These have important applications in the field of semiconductor chip fabrication, where microscopic defects at a surface layer make the difference between a good or bad chip. Holes or vias in an intermediate insulating layer often provide a physical conduit for an electrical connection between two outer conducting layers. If one of these holes or vias becomes clogged, it will be impossible to establish this electrical connection and the whole chip may fail. Examination of the microscopic defects in the surface of the semiconductor layers is necessary to ensure quality control of the chips.
Electron beams have several advantages over other mechanisms to examine samples. Light beams have an inherent resolution limit of about 100 nm-200 nm, but electron beams can investigate feature sizes as small as a few nanometers. Electron beams are manipulated fairly easily with electrostatic and electromagnetic elements, and are easier to produce and manipulate than x-rays.
Electron beams in semiconductor defect inspection do not produce as many false positives as optical beams. Optical beams are sensitive to problems of color noise and grain structures whereas electron beams are not. Oxide trenches and polysilicon lines are especially prone to false positives with optical beams due to grain structure.
A variety of approaches involving electron beams have been utilized for examining surface structure. In low-voltage scanning electron microscopy (SEM), a narrow beam of primary electrons is raster-scanned across the surface of a sample. Primary electrons in the scanning beam cause the sample surface to emit secondary electrons. Because the primary electrons in the beam of scanning electron microscopy are near a particular known electron energy (called `E.sub.2 `), there is no corresponding charge build-up problem in SEM, and the surface of the sample remains neutral. However, raster scanning a surface with scanning electron microscopy is slow because each pixel on the surface is collected sequentially. Moreover, a complex and expensive electron beam steering system is needed to control the beam pattern.
Another approach is called Photo-Electron Emission Microscopy (PEM or PEEM), in which photons are directed at the surface of a sample to be studied, and by the photoelectric effect, electrons are emitted from the surface. On an insulating surface, the emission of these electrons, however, produces a net positive charge on the sample surface since there is a net flux of electrons from the surface. The sample continues to charge positively until there are no emitted electrons, or electrical breakdown occurs. This charge build-up problem limits the utility of PEEM for imaging insulators.
Another method of examining surfaces with electron beams is known as Low Energy Electron Microscopy (LEEM), in which a relatively wide beam of low-energy electrons is directed to be incident upon the surface of the sample, and electrons reflected from the sample are detected. However, LEEM suffers from a similar charge build-up problem since electrons are directed at the sample surface, but not all of the electrons are energetic enough to leave the surface. In LEEM, negatively-charged electrons accumulate on the surface, which repels further electrons from striking the sample, resulting in distortions and shadowing of the surface.
Several prior art publications have discussed a variety of approaches using electron beams in microscopy, but none have determined how to do so with parallel imaging at the same time the charge build-up problem is eliminated. One of these approaches is described by Lee H. Veneklasen in "The Continuing Development of Low-Energy Electron Microscopy for Characterizing Surfaces," Review of Scientific Instruments, 63(12), December 1992, pages 5513 to 5532. Veneklasen notes generally that the LEEM electron potential difference between the source and sample can be adjusted between zero and a few keV, but he does not recognize the charging problem or propose a solution to it. Habliston et al., in "Photoelectron Imaging of Cells: Photoconductivity Extends the Range of Applicability," Biophysical Journal, Volume 69, October 1995, pages 1615 to 1624, describe a method of reducing sample charging in photoelectron imaging with ultraviolet light.
Thus, there remains a need for a method utilizing electrons beams to investigate sample surfaces that eliminates the charge build-up problem and increases the speed of examining large sample surfaces.